1. Field of the Invention
The present invention relates to a broadcast type satellite communication system, and, more particularly, to broadcast or point-to-multipoint satellite communication systems using a multiple fixed or scanning spot beam digital broadcast satellite operating in the time-division or frequency-division mode and employing on-board regeneration which provides a minimum power realization.
2. Description of the Prior Art
Communication satellite services generally fall into one of two broad categories: (a) fixed point-to-point, and (b) point-to-multipoint or broadcast. Fixed point-to-point service is characterized by interconnections between user pairs, that is, each message originating at a given ground station of the network is intended for reception only by one other ground station of the network. High capacity digital satellite systems employing multiple spot beam antennas and reusing the frequency band among all beams have been considered for such services, and Time Division Multiple Access (TDMA) techniques in conjunction with on-board beam switching provides for complete interconnection among beams. For fixed point-to-point service, these techniques minimize the radio-frequency (r.f.) power requirements both at the ground and on board the satellite since the high gain spot beam satellite antenna concentrates the radiation pattern between user pairs rather than radiating information over a broader region where reception is unintended. Moreover, since the individual beams are spatially isolated, the frequency band can be reused among the beams, thereby increasing the transmission capacity relative to wide area coverage systems. Extensions of these techniques to include a scanning spot beam satellite antenna, and multiple scanning spot beams have also been devised. In this regard see, for example, U.S. Pat. No. 4,188,578 issued to D. O. Reudink et al. on Feb. 12, 1980 and the article "Methods for Achieving High-Capacity Universal Service Satellites" by D. O. Reudink et al. in the NTC Conference Record, Birmingham, Ala., Dec. 3-6, 1978, Vol. 1 at pages 8.2.1-8.2.6.
By contrast, broadcast service is multidestinational in nature, that is, messages originating at some given ground station are intended for reception at a large number of ground stations geographically dispersed over some wide service area. Broadcast service might be used for distribution of television programming, video conferencing, or multipoint electronic mail. Typically, most ground stations of the network have an interest in receiving only a small percentage of the total number of up-link messages.
Satellites employing wide area coverage antenna patterns are often considered as candidates for broadcast networks since all receiving terminals are within the field of view of the antenna. The satellite, then, simply amplifies the arriving up-link signals and retransmits these on the down-link; each receiving terminal extracts those messages of interest, ignoring the remainder. In this regard see, for example, the article "Japanese Broadcast Satellite" by T. Ohtake et al. in the Microwave Journal, Vol. 20, No. 9, September, 1977, at pages 53-55. Such systems, however, do not provide the flexibility to efficiently accommodate dynamically varying broadcast traffic demands. For example, consider the distribution of network television programming via an area coverage broadcast satellite. The required number of transponders is dictated by the peak programming demand; when fewer simultaneous programs are to be distributed, a sizeable block of transponder resources are unused. Furthermore, the use of an area coverage antenna implies indiscriminate radiation of all messages over the entire service region regardless of intended destinations; the low antenna gain must therefore be compensated by allocating more r.f. power to each transponder. Finally, the total number of simultaneous programs that can be accommodated is limited to that provided by reusing the frequency band once in the orthogonal polarization.
The problem remaining in the prior art is to provide a broadcast satellite which can increase the flexibility and reduce the radiated power requirements over present type prior art broadcast satellites.